1. Technical Field of the Invention
The present invention relates generally to a spark plug for internal combustion engines which may be used in automotive vehicles, co-generation systems, or gas feed pumps, and more particularly to such a spark plug designed to ensure the stability of insulation resistance.
2. Background Art
FIGS. 13 and 14 illustrate a spark plug 9 as a typical example for use in internal combustion engines. The spark plug 9 includes a metal shell 94 with an external plug mounting thread 941, a porcelain insulator 92 retained inside the metal shell 94, a center electrode 93 disposed in the porcelain insulator 92, and a ground electrode 95 welded to the metal shell 94 to define a spark gap 911 between itself and the tip of the center electrode 93.
Usually, low-speed running of the engine may cause the engine to smolder, so that combustion of an air-fuel mixture produces carbon which is adhered to the surface of a nose 922 of the porcelain insulator 92. An increase in deposit of the carbon may create a conductive connection between the center electrode 93 and the metal shell 94, thus decreasing an dielectric resistance greatly therebetween. This may result in a failure in producing spark discharges in the spark gap 911. This tends to take place, especially in high heat range spark plugs) that is, spark plugs with a short insulator nose.
The above phenomenon found in the spark plug 9 will be analyzed below with reference to FIG. 15.
Carbon particles C are deposited in sequence from the tip to the base of the insulator nose 922 located inside the metal shell 94.
The insulator nose 922 of the spark plug 9 is, as can be seen in FIG. 14, shaped to have the diameter decreasing gradually from an outer annular shoulder 921 placed in abutment with an inner annular shoulder 942 of the metal shell 94 to the tip thereof. In other words, the insulator nose 922 is so designed that an air gap between itself and the inner wall of the metal shell 94 decreases from the tip thereof toward the outer annular shoulder 942. The increase in deposit of the carbon particles C on the insulator nose 922, therefore, results in a decrease in interval between the surface of a layer of the carbon particles C and the inner wall of the metal shell 94. When such an interval reaches a certain value, it will cause sparks (also called side sparks) to form between the outer surface of the insulator nose 922 and the metal shell 94.
Further increasing of the deposits of carbon particles C on the insulator nose 922 will result in the formation of an electrical connection between the center electrode 93 and the metal shell 94, thereby decreasing the insulation resistance greatly therebetween.
The deterioration in insulation resistance between the center electrode 93 and the metal shell 94 arising from smoldering of the engine may be alleviated by increasing the speed of the engine to elevate the temperature in the combustion chamber to burn off the carbon particles C settling on the surface of the porcelain insulator 92. The time of formation of the side sparks may be viewed as an indication of a suitable time when the carbon deposits is to be burned off. Specifically, the formation of the side sparks indicates the fact that a greater amount of carbon deposits have settled on the insulator nose 922, which also shows a suitable time when any measures should be taken to burn off the carbon deposits. The formation of the side sparks represents the occurrence of the so-called tracking (i.e., the creation of a conductive path through which the side sparks travel) within the engine which the vehicle operator usually perceives as mechanical vibrations of the engine.
The structure of the spark plug 9, however, has the problem in that the interval between the insulator nose 922 and the metal shell 94 decreases at a constant rate from the tip of the insulator nose 922 to the outer annular shoulder 921, thus causing a short circuit to be formed between the metal shell 94 and the center electrode 93 in a small amount of time after the side sparks are created, which leads to a difficulty for the vehicle operator to take measures to burn off the carbon deposits on the spark plug 9 after perceiving the occurrence of the tracking. There is also another problem that when the vehicle operator has perceived the tracking and stopped the engine, it may result in a failure in restarting the engine due to a great decrease in insulation resistance between the center electrode 93 and the metal shell 94 (i.e., the ground electrode 95) resulting from the deposit of carbon particles C on the insulator nose 922.
The shape of the insulator nose 922 having the diameter decreasing gradually from the outer annular shoulder 921 placed in abutment with the inner annular shoulder 942 of the metal shell 94 to the tip thereof results in a difficulty for the heat to be transferred or dissipated toward the base of the insulator nose 922. Therefore, a rapid elevation in temperature in the combustion chamber of the engine arising from, for example, sudden acceleration of the vehicle may cause the insulator nose 922 to be subjected to a great stress.
The poor transferring of the heat away from the insulator nose 922 will result in an increased temperature of the insulator nose 922. Thus, when the insulator nose 922 to which fuel is adhered is cooled rapidly, it will cause the insulator nose 922 to experience a great stress, which may lead to breakage thereof.
The above described deterioration of the insulation resistance between the center electrode 93 and the metal shell 94 due to the carbon deposit on the insulator nose 922 or the breakage of the insulator nose 922 tends to occur, especially in high-power engines mounted in tuned up cars.
Japanese Patent No. 2953227 teaches a park plug in which the base of the insulator nose 922 facing the inner annular shoulder 942 of the metal shell 94 is designed to have the width great enough to improve the dielectric strength thereof. This structure is, however, not designed to form the carbon deposit-caused side sparks promptly.